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| United States Patent |
6,037,583
|
|
Moehler
, et al.
|
March 14, 2000
|
Control system for a scanner drive
Abstract
A control system for a scanner is disclosed, especially for a laser
scanning microscope, with an oscillating motor for driving an oscillating
mirror serving for the linearly oscillating deflection of a beam bundle,
with a control unit for supplying the oscillating motor with an exciting
current which is variable with respect to the control frequency, frequency
curve, and amplitude, with a function generator which is connected with
the control unit, and with a measurement value transducer for obtaining a
sequence of information about the deflection positions of the oscillating
mirror.
The measurement value transducer is connected with the function generator
by way of a logic unit for determining correction values for the exciting
current. Accordingly, it is advantageously possible, by evaluating the
information supplied by the measurement value transducer about the actual
deflection position of the oscillating mirror, to determine correction
values with the assistance of the logic unit. These correction values can
be used, in turn, to influence the control frequencies emitted by the
function generator in such a way that deviations are minimized or
completely prevented.
| Inventors:
|
Moehler; Gunter (Jena, DE);
Schoeppe; Guenter (Jena, DE);
Tille; Sebastian (Marburg, DE)
|
| Assignee:
|
Carl Zeiss Jena GmbH (Jena, DE)
|
| Appl. No.:
|
010079 |
| Filed:
|
January 21, 1998 |
Foreign Application Priority Data
| Jan 27, 1997[DE] | 197 02 752 |
| Current U.S. Class: |
250/235; 250/230; 359/199; 359/221 |
| Intern'l Class: |
H01J 003/14 |
| Field of Search: |
250/234,235,236,214 R,230
359/198,199,212,213,221
358/494
|
References Cited
U.S. Patent Documents
| 4800270 | Jan., 1989 | Blais.
| |
| 5187364 | Feb., 1993 | Blais | 250/236.
|
Other References
GIT Fachz. Lab Sep. 1984, Article "Laser-Scan-Mikroskop--Aufbau und
Anwendungen" (Apparatus and Applications) V.Wilke and A. Siegel (pp.
765-766, 771-772).
Laser Magazin Mar. 1986, Article "Von Low-Cost bis High-Tech:
Laser--Scanner finden ihren Markt" K. Dickmann (pp. 10,12,14, 16 and 18).
R. Oldenbourg Verlag Muenchen Wein 1977, Elektronische Signalverarbeitung
"Mit 215 Bilden und 46 Tabellen" H. Vahidiek (pp. 220-221).
|
Primary Examiner: Westin; Edward P.
Assistant Examiner: Pyo; Kevin
Attorney, Agent or Firm: McAulay Nissen Goldberg Kiel & Hand, LLP
Claims
What is claimed is:
1. A control system for a scanner drive comprising:
an oscillating motor for driving an oscillating mirror for providing
linearly oscillating deflection of a beam bundle;
a control unit for supplying the oscillating motor with exciting current
which is variable with respect to the control frequency, triangular wave
and amplitude;
a function generator being connected with the control unit;
a measurement value transducer for obtaining a sequence of information
about the deflection positions of the oscillating mirror; and
a logic unit connecting the measurement value transducer with the function
generator for determining correction values for the exciting current, the
logic unit determining the values of k.sub.1 . . . k.sub.n and .phi..sub.1
. . . .phi..sub.n for the Fourier frequencies in the following series:
y=4/.pi.* [k.sub.1 sin(x+.phi..sub.1)-k.sub.2 sin(3x+.phi..sub.2)/3.sup.2
+k.sub.3 sin(5x+.phi..sub.3)/5.sup.2 . . . -+ . . .], where k.sub.1 to
k.sub.n represent the correction factors x represents the deflection
angle, and .phi..sub.1 to .phi..sub.n represent the phase angles.
2. The control system for a scanner drive according to claim 1, wherein at
least one signal output of the function generator is connected with an
associated signal output of the logic unit for conveying reference signals
and comparison signals via signal path.
3. The control system for a scanner drive according to claim 1, wherein the
logic unit converts the sequence of information about the deflection
positions of the oscillating mirror according to amplitude and phase of
the scanner drive with reference to a plurality of control frequencies to
provide corrected information.
4. A control system for a scanner drive comprising:
an oscillating motor for driving an oscillating mirror for providing
linearly oscillating deflection of a beam bundle;
a control unit for supplying the oscillating motor with exciting current
which is variable with respect to the control frequency triangular wave
and amplitude;
a function generator being connected with the control unit;
a measurement value transducer for obtaining a sequence of information
about the deflection positions of the oscillating mirror; and
a logic unit connecting the measurement value transducer with the function
generator for determining correction values for the exciting current,
the logic unit modelling a corrected control function from the comparison
of actually reached deflection positions with the desired deflection
position, wherein the modelling of a corrected control function is carried
out on the basis of correction values derived from this comparison, and
wherein coefficients k.sub.1 to approximately k.sub.5 are locked into the
control function for small phase errors and deviations .DELTA..phi..sub.1
to approximately .DELTA..phi..sub.5 are locked into the control function
for large phase errors.
5. The control system for a scanner drive according to claim 3, wherein the
corrected information provided by the logic unit are provided on the basis
of a Bode diagram.
6. The control system for a scanner drive according to claim 4, wherein the
logic unit determines correction values for the first to the twentieth
harmonic resonant frequency of the control frequency.
7. The control system for a scanner drive according to claim 1, wherein an
analog-to-digital converter is provided in the signal path between the
measurement value transducer and the logic unit, and a digital-to-analog
converter is provided in the signal path between the function generator
and the control unit for supplying the oscillating drive.
8. The control system for a scanner drive according to claim 7, wherein a
digital signal processor is provided as an analog-to-digital converter.
9. The control system for a scanner drive according to claim 1, wherein the
logic unit and the function generator are connected with a clock
generator.
10. The control system for a scanner drive according to claim 1, wherein a
galvanic drive is provided as oscillating drive.
11. The control system for a scanner drive according to claim 1, wherein a
capacitive angle measurement system is provided as the measurement value
transducer.
12. The control system for a scanner drive according to claim 11, wherein
the angle measurement system is designed for the detection of position
values of the oscillating mirror in bidirectional scanning direction and
bidirectional position values are present at the input of a logic unit via
the digital signal processor.
Description
BACKGROUND OF THE INVENTION
a) Field of the Invention
The invention is directed to a control system for a scanner drive,
especially for a laser scanning microscope, with an oscillating motor for
driving an oscillating mirror serving for the linearly oscillating
deflection of a beam bundle, with a control unit for supplying the
oscillating motor with exciting current which is variable with respect to
control frequency, frequency curve, and amplitude, with a function
generator which is connected with the control unit, and with a measurement
value transducer for obtaining a sequence of information about the
deflection positions of the oscillating mirror.
b) Description of the Related Art
Optical devices with scanning arrangements, including laser scanning
microscopes, are known in principle in the art. A laser which focusses
light along a beam path onto a small light point, generally called a
pixel, in a focal plane is typically used as a radiation source. In this
way, virtually all of the laser light is guided to this individual target
point.
The scanning device of an instrument of the kind mentioned above serves for
the linear deflection of the light coming from the laser as well as the
light reflected from the object plane and, in this respect, for moving the
light point in the image plane or in the object plane. A raster scanning
device which is controlled synchronously with the scanner emits the
resulting detector output signal as image information.
For oscillating deflection of the beam path, it is known to provide
electromechanically driven mirrors and to deflect the beam path in such a
way that the target point moves in the direction of an axis which will be
called the x-axis. For this purpose, the mirror can direct the laser
bundle onto a second mirror which is driven in the same way and which
causes a movement of the target point in the direction of an orthogonal
axis, the y-axis.
The deflection in the x-axis will be considered more closely in the
following. Although the deflecting mirrors that are used have smaller
dimensions and accordingly have less mass, the problem in such scanning
devices consists in always generating fast and accurate mirror movements
for the purpose of good image linearity with short image formation times.
This is because the mirror movement or beam path follows the drive signals
emitted by the control unit with only varying degrees of faithfulness due
to different interference influences. This is not adequate for a highly
efficient scanning device in which the demand for high scanning frequency
must always be met and in which it is required that the target point
maintains a constant speed over the entire deflection phase.
In order to obtain drive characteristics for the deflecting mirror which
are as linear as possible, a control signal with a triangular wave is
generated in the control unit. The phases and amplitude of a drive signal
of this kind form the basic precondition for approximation of the
deflection to linear movement of the target point depending on time.
It is known in the art to use harmonic analysis, i.e., the determination of
Fourier coefficients, for the purpose of the resultant approximation of a
triangular wave. A scanning device of this kind with associated control
unit is described, e.g., in DE-OS 4322694. In this case, control signals
are generated on the basis of two of the Fourier components, giving a
relatively good resultant approximation of a triangular wave. The type of
control shown in this case disadvantageously leads to unsatisfactory
results because the two frequencies are treated differently by the scanner
according to amplitude and phase. This is the case even when additional
harmonics of the fundamental frequency are used for additional correction.
In other words, the solution suggested in this case is not suitable for
realizing the desired linearization.
In the publication mentioned above, two resonant scanners and a
galvanometer scanner are provided for deflection of the laser beam in the
x-axis, wherein the galvanometer scanner is used to superpose a DC
oscillating movement on the resonant movement supplied by the resonant
scanners. As is well known, the oscillating movement of a resonant scanner
is caused to a great extent by the exchange of energy between the motion
of a mass, especially the mirror, and the deflection of an elastic
element, such as a spring, to which the mass is attached.
In a departure from the construction described thus far according to which
a plurality of separate scanners are operated within the scanning device,
each with its own resonant frequency, it is known to operate an individual
scanner with a plurality of resonant frequencies. For example, U.S. Pat.
No. 4,859,846 describes the operation of a scanner which works with a
mirror and generates a plurality of resonant frequencies for this scanner
by means of a suitable control system. This system is also a resonant
scanner system. This solution is also unsuitable for overcoming the
disadvantage that the actual deflection position is falsified by the
position predetermined by the control signal because of various
interfering influences, e.g., external temperature influences, influencing
variables associated with material, etc.
OBJECT AND SUMMARY OF THE INVENTION
Therefore, the primary object of the invention is to further develop the
control system for a scanner drive of the kind mentioned above in such a
way that the accuracy of the actual deflection position and of the
linearity of the deflection is increased while retaining the advantageous
generation of a control signal based on a triangular wave.
According to the invention, this object is met in that the measurement
value transducer is connected with the function generator by way of a
logic unit for determining correction values for the exciting current.
Accordingly, it is advantageously possible, by evaluating the information
supplied by the measurement value transducer about the actual deflection
position of the oscillating mirror, to determine correction values with
the assistance of the logic unit. These correction values can be used, in
turn, to influence the control frequencies emitted by the function
generator in such a way that deviations are minimized or completely
prevented. In this respect, the solution according to the invention
provides a control of the scanning movement which detects deviations of
the actual deflection position of the mirror from the position provided by
the control frequency and exerts influence on the further controlling of
the oscillating mirror by appropriately changing the exciting current.
In an advantageous construction of the invention, at least one signal
output of the function generator is connected with an associated signal
output of the logic unit for conveying reference signals and comparison
signals. In this way, it is ensured that the control frequencies serving
as a basis for a comparison with the actual deflection of the mirror and
with the response frequency of the mirror to the control frequency are
also available in the logic unit.
A first computing circuit for converting the information about the
deflection positions of the oscillating mirror according to amplitude and
phase of the scanner drive with reference to a plurality of control
frequencies should be provided in the logic unit. The results can be
represented in the form of a Bode diagram.
Further, a second computing circuit should be provided in the logic unit
for determining the values of k.sub.1 . . . k.sub.n and .phi..sub.1, . . .
.phi..sub.n for the Fourier frequencies in the following series:
y=4/n * [k.sub.1 sin(x+.phi..sub.1 (p.sub.1)-k.sub.2
sin(3x+.phi..sub.2)3.sup.2 +k.sub.3 sin(5x+.phi..sub.3) /5.sup.2 -+. . .
],
where k.sub.1 to k.sub.n represent the correction factors, x represents the
deflection angle, and .phi..sub.1 to .phi..sub.n represent the phase
angles. This computing circuit makes possible a harmonic analysis of the
response movement of the deflecting mirror and the determination of the
Fourier coefficients k.sub.1 to k.sub.n. For this purpose, the response
frequency is broken down into a sum of pure oscillations (harmonic
oscillations) and a constant component.
Further, a third computing circuit should be provided in the logic unit for
modelling a corrected control function from the comparison of actually
reached deflection positions with the desired deflection position. The
modelling of a corrected control function is carried out on the basis of
correction values derived from this comparison. For this purpose, the
coefficients k.sub.1 to approximately k.sub.5 are locked into the control
function for small phase errors and deviations .DELTA..phi..sub.1 to
approximately .DELTA..phi..sub.5 are locked into the control function for
large phase errors.
The corrected control commands which are calculated by taking into account
the correction values in the logic unit are compiled in corresponding data
sets, sent to the function generator, and initially stored therein. A
highly precise correction of the control frequency is ensured by taking
into account in this way the correction values for the first to the fifth
resonant frequency.
In a further preferred construction of the invention, an analog-to-digital
converter is provided in the signal path between the measurement value
transducer and the logic unit, and a digital-to-analog converter is
provided in the signal path between the function generator and the control
unit for supplying the oscillating drive. A digital signal processor can
be provided as an analog-to-digital converter. This ensures a conversion
of the analog signals sent by the measurement value transducer into the
digital signals required by the logic unit and, correspondingly, a
conversion of the digital frequency signals sent by the function generator
into analog signals for preparation for the control unit.
Further, in an advantageous manner, the logic unit, the function generator,
the analog-to-digital converter, and the digital-to-analog converter
should each be connected with a clock generator. In this way, it is
possible to send the information supplied by the measurement value
transducer about the response frequency to the logic unit and the
corrected control commands to the function generator and the control
commands for the following scanning process in a synchronized manner.
A galvanic drive should be provided as oscillating motor. In this way, a
defined oscillating movement of the oscillating mirror provided by the
exciting energy can be realized. A capacitive angle measurement system
should be provided as measurement value transducer. This angle measurement
system should be designed in such a way that it is configured for the
detection of position values of the oscillating mirror in both scanning
directions, i.e., for both the forward movement and returning movement of
the galvanic drive. This has the advantageous result that the
bidirectional position values are present at the input of the logic unit
via the digital signal processor, and the image formation time can thus be
reduced by approximately half in comparison with a scanning process in
only one direction, i.e., the forward movement and returning movement of
the scanner can be utilized; smaller deviations which may occur can be
made identical for the forward movement and the returning movement.
The invention is explained more fully hereinafter with reference to an
embodiment example.
BRIEF DESCRIPTION OF THE DRAWINGS
In the drawings:
FIG. 1 a schematic wiring diagram of the control system according to the
invention;
FIG. 2 the shape of an uncorrected control voltage for an image formation
time <1s; and
FIG. 3 the shape of the corrected control voltage for the image formation
time <1s.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
According to the embodiment example shown in FIG. 1, a galvanic drive 2, an
oscillating mirror 3 which is coupled with the galvanic drive 2 by a
mechanical connection 9, a control unit 4 whose output communicates with
the control input of the galvanic drive 2, and a capacitive angle
measurement system 7 which serves to determine the phase-dependent mirror
position are provided in a scanner 1. Further, there is a function
generator 5 which is connected with the signal input of a digital signal
processor 6 via a signal path 16, while the output of the signal processor
6 is applied to the command input of the control unit 4 via a signal path
17. A first output of the capacitive angle measurement system 7 is
connected, via a regulating path 8, with a control input of the control
unit 4.
The control unit 4 is designed in such a way that it supplies the galvanic
drive 2 with an exciting current which is variable with respect to its
frequency, oscillation shape, and amplitude. The function generator 5 is
designed in such a way that it can generate a plurality of different
frequencies which can be impressed in the control unit 4 on the control
voltage for the galvanic drive 2 via the signal path 16, the digital
signal processor 6, and the signal path 17 in an individually selected
manner. The control voltage is based on a synthetic mesh voltage or delta
voltage (see FIG. 2 and FIG. 3) which contains only frequencies that can
be processed by the galvanic drive 2. Specifically, it is assumed, for
example, that forty-two different frequencies up to a maximum of 5 kHz are
available to be called up in the frequency generator 5.
During the operation of this arrangement, the galvanic drive 2 transmits
every frequency contained in the exciting current or in the control
voltage to the oscillating mirror 3 via the mechanical connection 9, since
the fundamental frequency and all harmonics produce a response which
changes with the respective gain and phase displacement and which
expresses itself in a correspondingly changed deflection position of the
oscillating mirror 3, wherein the respective deflection position
corresponds to a position of the laser beam in its path over a line
scanned in the x-direction. It is assumed by way of example that 1,200
deflection positions are to be scanned on every path along the
x-direction, wherein one picture point in the object plane is assigned to
each deflection position.
The deflection position occupied by the oscillating mirror 3 in each
instance corresponds to a position value which is represented by the
capacitive angle measurement system 7 and which is supplied to the control
unit 4 over the regulating path 8 and, in case of a discrepancy between
the reference value and the actual value of the mirror position from the
predetermined or ideally desired deflection position, is immediately used
in the control unit 4 for correcting the control signal for the subsequent
controlling of the galvanic drive 2. This process, known per se,
corresponds to a conventional regulation.
However, in order to realize very short image formation times, especially
in the range of less than 1s, the synthetic delta voltage which is made
available and which initiates the scanning movement must be adapted as far
as possible to the response behavior of the scanner according to amplitude
and phase, so that a highly precise deflection of the oscillating mirror 3
is ensured based on the control voltage shape. This means that the
transmission factor of the control function with respect to the response
movement must be approximated to the value of 1 as far as possible and the
deviation between the control function and response movement must
accordingly be limited to a minimum, for example, <0.5 pixels. In order to
achieve this, the control system shown up to this point, which is based on
regulation of the control frequency, is supplemented according to the
invention by a logic unit 13 whose command input is connected, via signal
path 12, a second digital signal processor 11, and signal path 10, with a
second output of the capacitive angle measurement system 7. The output of
the logic unit 13 is connected via a signal path 14 with a control input
of the function generator 5. An additional coupling between the function
generator 5 and the logic unit 13 is formed by the signal path 15 for
transmitting reference signals and comparison signals from the function
generator 5 to the logic unit 13. Further, a clock generator 18 is
provided, which clock generator 18 is connected with the second digital
signal processor 11 via signal path 19, with the logic unit 13 via signal
path 20, with the function generator 5 via signal path 24, and with the
first digital signal processor 6 via signal path 21.
Before starting the actual scanning operation, for example, in a laser
scanning microscope, this circuit arrangement can be used first to test
the entire control system for system errors and to calibrate it while
taking into account system errors in such a way that a highly accurate
deflection of the oscillating mirror 3 is possible depending on the
predetermined frequency. For the purpose of this process referred to as
calibration, all of the frequencies prepared 42 by the function generator
5 are first called up one after the other and scanning processes are
initiated with these frequencies. For this purpose, the digital signal
processors 6 and 11, the function generator 5, and the logic unit 13 are
synchronized by the clock generator 18. The response received by the logic
unit 13 is evaluated and analyzed in the form of a Bode diagram, wherein
the Bode diagram enables the determination of a phase angle and an
associated transmission factor for every frequency of the Fourier
coefficients. Based on the determined phase angle and the transmission
factors, it is possible to synthesize control functions for scanner
frequencies within a wide range (1/64 Hz . . . .apprxeq.600 Hz) which are
utilized for a corrected control frequency for the galvanic drive 2 with
that of the capacitive angle measurement system 7. The data sets which are
synthesized in this way for an oscillation and triangular wave are stored
in the function generator 5 and can be called up from the latter
cyclically. In this way, data sets which take into account the
characteristics of the scanning system are available in the function
generator 5 as a result of the calibration step. These data sets determine
how the scanning drive is to be controlled in order to obtain the desired
highly precise periodic deflection.
Further, by means of the accurate scanning operation which is now possible,
deviations from the ideal deflection position are determined and corrected
control commands are derived therefrom and stored in the function
generator for each of the 1,200 individual deflection points of the laser
beam by evaluation of the information reported by the capacitive angle
measurement system 7 analogous to the above-described calibration step.
Subsequently, corresponding to the clock frequency determined by the clock
generator, inquiry and further processing of the corrected control data
sets is carried out cyclically for achieving highly precise scanning
positions.
FIG. 2 shows the uncorrected control voltage for a specific system in the
form of a synthetic delta voltage for the scanning process with an image
formation time of 0.75 s. The length z shows the dimensioning of a line to
be scanned in the x-direction. Further the triangular wave 22 for the
control voltage and the triangular wave 23 for the response movement are
shown. It can be seen that the triangular wave 23 does not have its zero
crossing at z/2, i.e., the oscillating mirror 3 and accordingly the
deflected laser beam do not exactly follow the control voltage
predetermined by the triangular wave 22.
FIG. 3 shows the situation after correction has been carried out. The
triangular wave 22 of the response movement is smoothed in particular at
the edge near the reversal points and, moreover, now has its zero crossing
exactly at z/2.
Synthetic delta voltages having a high mirror symmetry can be realized with
this circuit arrangement according to the invention and, consequently,
bidirectional scanning with the highest accuracy requirements is made
possible.
While the foregoing description and drawings represent the present
invention, it will be obvious to those skilled in the art that various
changes may be made therein without departing from the true spirit and
scope of the present invention.
Reference Numbers
1 scanner
2 oscillating drive
3 oscillating mirror
4 control unit
5 frequency generator
6 first digital signal processor
7 capacitive angle measurement system
8 regulating path
9 mechanical connection
10 output of the angle measurement system
11 second digital signal processor
12 signal path
13 logic unit
14, 15, 16, 17, 19, 20, 21, 24 signal paths
18 clock generator
22 triangular wave control frequency
23 triangular wave response frequency
z line length
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